1. Frequency Control Turbine Governor Droop NERC Requirement
Droop Setting and Turbine Response
Governor underperformance
ERCOT Droop
Size does make a difference
Sydney Niemeyer

2. Turbine Governor Droop, Speed Regulation or Speed Error are common terms used in describing a turbine’s response to Changes in Interconnection Frequency (speed).
Not the same as the “Regulation” Ancillary Service Deployment of ERCOT’s Frequency Control System.
NERC requires all generators greater than 10 MW’s to have an operating governor.
Droop distributes Frequency regulation to all generators in the interconnect.
Recommended droop settings of 4 to 5% with a maximum dead band of +/ Hz.

3. Droop Setting Determines Response
5% Droop: 100% change in generator output for a 5% change in Frequency or Speed.
A 5% change in frequency, 60 Hz x 0.05 = 3 Hz or for a 2 pole generator, 3600 rpm x 0.05 = 180 rpm.
4% Droop: 100% change in generator output for a 4% change in Frequency or Speed.
A 4% change in frequency, 60 Hz x 0.04 = 2.4 Hz or for a 2 pole generator, 3600 rpm x 0.04 = 144 rpm.
4% Droop setting is more sensitive (responsive) than the 5% Droop setting.

4. Example of Expected Response
150 MW unit at 5% Droop
150 / 3 Hz = MW/Hz or in tenths of Hz, 5.00 MW/0.1 Hz
Frequency change from to should result in the generator increasing output 5.0 MW’s
150 MW unit at 4% Droop
150 / 2.4 Hz = MW/Hz or in tenths of Hz, 6.25 MW/0.1 Hz
Frequency change from to should result in the generator increasing output 6.25 MW’s

5. Expected Governor Response
5 % Droop:
Unit Net Capability/30 = MW/0.10 Hz
4 % Droop:
Unit Net Capability/24 = MW/0.10 Hz

6. 4% 5% 5% 4%

7. Testing Governor Performance
ERCOT requires off line testing of Steam Turbine governors every 2 years.
Mechanical and Electro hydraulic test forms are in the Operating Guides.
Combustion Turbines do not have an approved test procedure.
On Line observation verifies proper performance for limited frequency range.

9. Droop Underperformance due to Unit Control Mode – Combustion Turbine
Base Load with output limited by Exhaust Temperature – Small governor response then reverses and reduces output.
Pre-Selected Load set-point, limited by Exhaust Temperature – some governor response within Load set-point dead-band.
Partial Load, not controlling to a load set-point – Full governor response.

10. Control Mode of Combustion Turbine May Limit Governor Response
Actual Shaft Speed
3600 RPM Reference
FSR - Full Speed Regulation
Speed Error
Fuel Control Valve
Droop Setting
Exhaust Temperature Limit
Pre-Select Control Mode
Pre-Select Load Set-point
Generator Output MW

11. Combustion Turbine in Pre-Select Mode – Limited Governor Response

12. Combustion Turbine in Partial Load Mode – Full Governor Response

16. ERCOT System Droop Unit mix determines Frequency Response
Possible Combination

17. ERCOT Load Contributes to System Droop
As Frequency decreases, load decreases. As Frequency increases, load increases.
Droop contribution of load %/MW of ERCOT Load, in MW/0.1 Hz.
34,700 MW Load x = 92.3 MW/0.1Hz.
Total ERCOT Droop includes Turbine droop and Load droop.
= MW/0.1Hz.
For a disturbance of 1250 MW’s and a system response of MW/0.1 Hz, System Frequency would decline by Hz. If starting at 60.00, would result in a minimum frequency of Hz.

18. 55,000 MW Load
20,000 MW Load

19. ERCOT’s Future Droop? What will the future unit mix be?
2000 MW’s of Wind Generation?
Combined Cycle Combustion Turbines want to be 100% loaded on Temperature Control.
What governor response the turbines don’t provide, the load will (at the expense of larger frequency deviations).

20. Increasing Wind & Combined Cycle Generation

21. ERCOT System Droop
Changing the unit mix changes minimum frequency for the same event.
Droop contribution of load %/MW of ERCOT Load, in MW/0.1 Hz.
34,700 MW Load x = 92.3 MW/0.1Hz.
Total ERCOT Droop includes Turbine droop and Load droop.
= MW/0.1Hz.
For a disturbance of 1250 MW’s and a system response of MW/0.1 Hz, System Frequency would decline by Hz. If starting at 60.00, would result in a minimum frequency of Hz.

22. Firm Load Shedding 59.30 Hz – Trips 5% Firm Load

23. Size Does Make a Difference
Eastern Interconnect and Western Interconnect Frequency is much more stable than ERCOT’s.
Based on 2001 Peak Planning Data and Load’s contribution to System Droop:
East 527,000 MW 1370 MW’s/0.1 Hz
West 132,492 MW 344 MW’s/0.1 Hz
ERCOT 53,391 MW 139 MW’s/0.1 Hz

24. ERCOT
WEST
Interconnection
Frequency
EAST